Essentially they are the same. Using natural seawater can be done, I have done it myself many years ago. You have to be careful where you collect it, preferably well offshore in the "blue water" zone, on the front of a boat not near a marina or beach where you might encounter engine fuel or other pollutants. Then cure it for a month or so in total darkness to kill any pathogens. I had good results, in fact my inverts seemed to like it, but I cannot say with any scientific authority that it was better than the artificial mix. With the high quality artificial mixes available today it's not worth the hassle and risk of pollutants in my opinion.

More importantly, you should be making sure your freshwater source (tapwater for many) is the best it can possibly be, and does not contain any excess phosphates or organics which can lead to unwanted algae in your marine system. Some tapwater sources contain a surprising amount of copper and other chemicals. For those with sensitive reef systems or invertebrates, this can be a big issue. The severity of these problems will differ from one city to the next, and leads many aquarists to purchase a reverse osmosis and deionization system (RO/DI), combined with carbon or Polyfilter products._________________Keepin' marines happy for 25 years